JP2658818B2 - Birefringent diffraction grating polarizer and optical head device - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a birefringent diffraction grating polarizer used for a light source module for optical fiber communication, an optical head for an optical disk, and an optical head device using the same.

[0002]

2. Description of the Related Art Polarizers, particularly polarizing beam splitters,
This is an element that obtains a specific polarization by making the propagation direction of light different between orthogonal polarizations. Conventionally, as a polarization beam splitter, utilizing the difference in transmission or reflection due to polarized light on the bonding surface of crystals with large birefringence, such as Glan-Thompson prism and lotion prism,
A prism that separates the optical path or a prism that is made of an isotropic optical medium such as glass is provided with a dielectric multilayer film on the bonding surface of the combined beam splitter, and uses the difference in interference due to polarization of the dielectric multilayer film. Those that reflect or transmit light are often used. However, these devices have drawbacks such as large size, low productivity, and high price. In addition, when the bulk type polarizer has only a polarizer function and is used for an optical disk optical head, it is difficult to combine a focus error detection function and a track error detection function, and it is difficult to reduce the size of the optical head.

On the other hand, in recent years, as a polarizer which is compact and has high productivity and which can combine functions other than the polarizer function, Japanese Patent Laid-Open No.
A birefringent diffraction grating polarizer described in 3-314502 is known. FIG. 7 is a sectional view of the birefringent diffraction grating polarizer. When the X-plate or Y-plate of the lithium niobate substrate 1 which is a birefringent crystal is subjected to proton exchange with benzoic acid, for example, light of a wavelength of 0.78 μm, which is generally used in an optical disc device, is exposed to the substrate. The refractive index for extraordinary light that is polarized light parallel to the crystal optical axis increases by about 0.12, and the refractive index for ordinary light that is polarized light perpendicular to the optical axis decreases by about 0.04. Therefore, a grating in which the exchange region 2 subjected to proton exchange and the non-exchange region not subjected to proton exchange are periodically arranged to function as a diffraction grating. When a phase compensation film 3 having an appropriate thickness is formed on the exchange region 2 to cancel the phase difference between ordinary light passing through the exchange region 2 and ordinary light passing through the non-exchange region on this grating, The grating does not act as a diffraction grating and can transmit ordinary light without diffracting. In other words, this lattice looks like a mere transparent substrate. When the phase difference with respect to the extraordinary light is π and the width of the region not exchanged with the proton exchange region 2 is equal by changing the depth of the exchange region 2 while satisfying the phase difference canceling condition with respect to the ordinary light, the extraordinary light Is completely diffracted. These phase relationships are given by:

[0004] _{{Δn e · T p + (} n d -n out) · T
_{d} · 2π / λ = π} {Δn o · T p + (n d -n out) · T d} · 2π /
λ = 0 where n _d and T _d are the refractive index and thickness of the phase compensation film 3,
_p is the depth of the proton exchange region 2, Δn _e , Δn _o are the refractive index changes of extraordinary light and ordinary light in the proton region 2, and n _out is the refractive index outside the hologram substrate, that is, n _{out in the} air layer.
= 1. Λ is the wavelength of light.

Further, the proton exchange regions 2 and the phase compensation films 3 are alternately arranged, and the phase difference between the light passing through the proton exchange regions 2 and the light passing through the dielectric phase compensation film 3 with respect to the extraordinary light is 0, For ordinary light, the polarization function can also be realized by setting the phase difference to π and making the width of the proton exchange region 2 and the phase compensation film 3 equal. In this case, the extraordinary light is transmitted without being diffracted, and the ordinary light is completely diffracted.

Japanese Patent Application Laid-Open No. 3-2913 discloses an example of using this birefringent diffraction grating polarizer in an optical head device.
Japanese Patent Application Laid-Open No. 7-29129 and a hologram element described in JP-A-3-29129. While these hologram elements have the cross-sectional structure shown in FIG. 7, a grating pattern is formed from a plurality of grating regions having different diffraction directions as shown in FIG. 8 to detect a focus error signal and a track error signal of the optical head. ing. As a similar grid pattern, there is a pattern in which a grid area is divided as shown in FIG.

FIG. 10 shows an optical head device for a video disk, a write-once optical disk and a rewritable phase-change optical disk described in Japanese Patent Application Laid-Open No. 3-29129. The light emitted from the semiconductor laser 10 enters the hologram element 16 as ordinary light, transmits without being diffracted, passes through the collimating lens 11, the quarter-wave plate 13, and the objective lens 12, and
The light is focused on the optical disk 14. The return light from the optical disk 14 passes through the reverse path and reenters the hologram element 16. This return light has its polarization plane rotated by 90 degrees with respect to the original polarization by the quarter-wave plate 13 and is completely diffracted because it enters the hologram element 16 as extraordinary light. The -1st-order diffracted light 51 is received by the first photodetector 30 and the second photodetector 31. FIG. 11 is a diagram illustrating a state of diffracted light incident on the photodetector. The diffracted light from the regions A5 and B6 on the hologram in FIG. The focus error signal is shown in FIG.
The division line 9 on the hologram pattern functions as a knife edge, and is detected from the diffracted light by the double knife edge method. The track error signal is the convergence point 42, 43 in FIG.
Is detected by the push-pull method from the difference between the diffracted light from the region C7 and the diffracted light from the region D8. The recording signal is transmitted to the second photodetector 31.
, Or the sum of the amounts of light received by the first photodetector 30 and the second photodetector 31. Further, by receiving high-order diffracted light, the signal amount can be further increased.

FIG. 12 shows an optical head device for a magneto-optical disk described in Japanese Patent Application Laid-Open No. 3-29137. Light emitted from the semiconductor laser 10 passes through the collimating lens 11, the polarizing beam splitter 18, and the objective lens 12, and is then focused on the magneto-optical disk 15. The return light from the magneto-optical disk 15 follows the reverse optical path, is separated off the optical axis by the polarization beam splitter 18, becomes convergent light by the lens 19, and the extraordinary light component is diffracted by the hologram element 17, and the first,
The light is received by the second and third photodetectors 32, 33 and 34. The polarization beam splitter 18 receives a P beam from the semiconductor laser 10.
It has a polarization characteristic of partially reflecting and transmitting polarized light, and reflecting all S-polarized light orthogonal thereto, that is, all of the slight polarized light generated by the Kerr effect at the time of reflection of the magneto-optical disk 15. The recording signal is transferred to the holobram element 17 using the differential detection method.
From the 0th-order diffracted light and the 1st-order diffracted light, which are polarized and separated by the polarizer function. + 1st-order diffracted light 52 and -1st-order diffracted light 5
When detecting from the difference between the sum of 4 and the zero-order diffracted light 53, if the polarization direction of the return light and the crystal optical axis of the hologram element form an angle of about 42 degrees, the light quantity balances and the recording signal can be detected with small noise. . -1st-order diffracted light 54 and 0th-order diffracted light 53
In the case of detecting from the difference, the angle may be set to about 32 degrees.
FIG. 13 is a diagram for explaining the state of the diffracted light incident on the photodetector. The focus and track error signals are detected in the same manner as in FIG. 11 using the + 1st-order diffracted light 52 incident on the first photodetector 32.

[0009]

The above-mentioned conventional birefringent diffraction grating polarizer is of a transmission type, and therefore requires a deep proton exchange region and a thick phase compensation film to form a grating layer. Production time was required. Further, when this element is used in the above-mentioned conventional optical head device, it is difficult to further reduce the size. An object of the present invention is to provide a birefringent diffraction grating polarizer capable of forming a grating layer in a short time and an optical head device using the same, which can be further reduced in size.

[0010]

SUMMARY OF THE INVENTION A birefringent diffraction grating polarizer of the present invention has a surface of a diffraction grating comprising an ion exchange region and a dielectric pair periodically provided on a main surface of a crystal plate having optical anisotropy. A total reflection film is formed on the substrate.

A polarizing beam splitter layer is provided on the entire surface or a part of the surface opposite to the diffraction grating.

An optical head device according to the present invention includes a light source, an imaging lens system for focusing light from the light source on a recording medium, and a polarization of light returned from the recording medium orthogonal to a polarization of light from the light source. A quarter-wave plate that converts the light from the light source to the imaging lens system and diffracts the return light, and a photodetector that receives the diffracted light. The hologram element is characterized in that a total reflection film is formed on the surface of a diffraction grating composed of a dielectric pair and an ion exchange region periodically provided on a main surface of a crystal plate having optical anisotropy.

An optical head device according to the present invention includes a light source, an imaging lens system for converging light from the light source onto a recording medium, and reflecting light from the light source to the imaging lens system to return from the recording medium. A hologram element that diffracts and reflects light; and a photodetector that receives the diffracted and reflected light, wherein the hologram element is an ion that is periodically provided on a main surface of a crystal plate having optical anisotropy. A total reflection film is formed on a surface of a diffraction grating composed of an exchange region and a dielectric, and a polarization beam splitter layer is provided on a part of a surface opposite to the diffraction grating.

An optical head device according to the present invention includes a light source, an imaging lens system for focusing light from the light source onto a recording medium, and a part of the light from the light source reflected to the imaging lens system. A polarization beam splitter that transmits a part of the return light from the recording medium, a hologram element that reflects and diffracts the return light from the recording medium that has passed through the polarization beam splitter, and a light detector that receives the reflected and diffracted light The polarizing beam splitter has a polarizing beam splitter layer formed in a partial region of a plane where polarization is separated, and the hologram element is periodically arranged on a main surface of a crystal plate having optical anisotropy. It is characterized in that a total reflection film is formed on the surface of the diffraction grating formed of the ion exchange region and the dielectric provided, and the opposite surface of the diffraction grating and the polarization splitting surface of the polarization beam splitter are bonded. To.

[0015]

The operation and principle of the birefringent diffraction grating polarizer of the present invention are as follows. The birefringent diffraction grating polarizer of the present invention has a reflection type structure as shown in FIG. In this structure, since light reciprocates in the lattice layer, the required thickness of the lattice layer of the proton exchange region 2 and the phase compensation film 3 can be reduced to half or less as compared with the conventional transmission type structure. As described in the related art, the extraordinary light is diffracted and the ordinary light is reflected. The phase difference between the light passing through the proton exchange region 2 and the light passing through the region without proton exchange is π for the extraordinary light and 0 for the ordinary light. Should be fine. The phase relationship at that time is expressed by the following equation.

[0016] _{(Δn e · 2T p + '} n d · 2T d') · 2π / λ = π (Δn o · 2T p + 'n d · 2T d') · 2π / λ = o This T _{p, Td} is obtained as follows.

[0017] _{T p '= λ / {4} · (Δn e -Δn o)} T d' = -Δn o / n d · T p ' phase compensation depth and the dielectric of the proton exchange region 2 at this time The relationship between the thickness of the film 3 and the transmission type described in the above-mentioned prior art is as follows.

T _p ′ = T _p / 2 T _d ′ <T _d / 2 When light is obliquely incident on the grating, the optical path length is longer than in the case of normal incidence, so that the proton exchange region 2 and the phase compensation film are further increased. 3 can be made thinner.

[0019]

1 is a sectional view of a first embodiment of a birefringent diffraction grating polarizer according to the present invention. As the phase compensation film 3, a dielectric film having a refractive index of about 2.2 which is almost the same as that of the lithium niobate substrate 1 is used. For light having a wavelength of 0.78 μm, the thickness of the phase compensation film is about 800 n in the conventional transmission type structure.
m, the depth of the proton exchange layer is required to be about 2.4 μm. In the reflection type structure of the present invention, a polarizer is provided by a phase compensation film having a thickness of about 200 nm and a proton exchange region having a depth of about 1.2 μm. Function can be realized. As this dielectric film, Nb ₂
O ₅ , TiO ₂ , Ta ₂ O ₅ and the like. The total reflection film 4 includes a metal film or a dielectric multilayer reflection film.

FIG. 2 is a sectional view showing a birefringent diffraction grating polarizer according to a second embodiment of the present invention. As described in the description of the prior art, the transmission type structure of the birefringent diffraction grating polarizer has a structure in which proton exchange regions and dielectric phase compensation films are alternately arranged as described in JP-A-4-292853. Yes, the embodiment of FIG. 2 is a reflection type.

FIG. 3A shows a first example of the optical head device according to the present invention.
The optical head device for a video disk, a write-once optical disk, and a rewritable phase-change optical disk. The hologram element 20 has a reflective birefringent diffraction grating polarizer having a back surface as described above, and its grating pattern is the same as that shown in FIG. 8 or FIG. The pattern shown in FIG.
The light from the semiconductor laser 10 as a light source is
Incident on the grating layer with polarized light that is reflected without being diffracted by this grating layer. The reflected light is collimated by a collimating lens 11, passes through a 波長 wavelength plate 13 and an objective lens 12, and is then focused on an optical disk 14. The return light reflected by the optical disc 14 passes through the reverse optical path and is
Re-enter at 0. The polarized light at this time is rotated by 90 degrees from the original polarization direction by the reciprocating passage of the quarter-wave plate 13 and is diffracted by the hologram element 20.
And are received by the second photodetectors 35 and 36. FIG. 3 (b)
Indicates the state of the diffracted light at each photodetector, and detects the focus, track error signal, and recording signal in the same manner as in the method described in the related art.

FIG. 4A shows a second example of the optical head device according to the present invention.
In this embodiment, the semiconductor laser and the photodetector of the first embodiment are integrated in a module 60. FIG.
(B) is a diagram of the module 60. Light emitted from the semiconductor laser chip 25 is reflected by the mirror 24 and travels to the hologram element 20. The hologram element 20
Return light enters the photodetector group 27, and the focus and track error signal and the recording signal are detected as described above.

FIG. 5A shows a third example of the optical head device according to the present invention.
FIG. 4 is a diagram of an embodiment of the present invention, showing an optical head device for a magneto-optical disk. This hologram element 21 has a reflection grating layer 22
The polarization beam splitter layer 23 is formed in a partial area on the surface. This reflection grating layer 22 has the structure of the above-mentioned reflection type birefringent diffraction grating polarizer, and its grating pattern is the same as that shown in FIG. 8 or FIG. The pattern shown in FIG.
The polarization beam splitter layer 23 on the surface has a polarization characteristic of transmitting almost specific polarized light and transmitting and reflecting polarized light orthogonal to the polarized light at a constant light amount ratio, and forming a dielectric multilayer film on the surface or as described above. It can be formed by attaching a polarization beam splitter substrate having polarization characteristics to the surface. The light from the semiconductor laser 10 as the light source enters the polarized light partially reflected by the polarizing beam splitter layer 23 of the hologram element 21. The reflected light is collimated by a collimating lens 11 and is reflected by an objective lens 12.
Is condensed on the magneto-optical disk 15. The return light reflected by the magneto-optical disk 15 reenters the hologram element 21 through the opposite optical path. In the polarization beam splitter layer 23, a part of the original polarized light of the return light is transmitted, and the polarized light orthogonal thereto, that is, the slight polarized light generated by the Kerr effect at the time of reflection on the surface of the magneto-optical disk 15 is almost transmitted. The light transmitted through the polarization beam splitter layer 23 is diffracted and reflected by the reflection grating layer 22 on the back surface, and the diffracted light and the reflected light are received by the photodetector 37.

FIG. 5B shows the state of the diffracted light at the photodetector 37. Focus and track error signals are detected from the + 1st-order diffracted light in the same manner as described in the prior art. By setting the polarization direction of the return light and the crystal optic axis of the hologram element 21 at an appropriate angle as described in the prior art, the recording signal is also used for the 0th-order diffraction light and the -1st-order diffraction light or the 0th-order diffraction light. And ± 1st order diffracted light. It is also possible to increase the recording signal by receiving higher-order diffracted light.

FIG. 6A shows a fourth example of the optical head device according to the present invention.
In this embodiment, the semiconductor laser and the photodetector of the third embodiment are integrated in a module 61. FIG.
(B) is a diagram of the module 61. Light emitted from the semiconductor laser chip 25 is reflected by the mirror 24 and travels to the hologram element 21. The return light from the hologram element is incident on the photodetector group 26, and the focus and track error signal and the recording signal are detected as described above.

FIG. 14 is a view of a fifth embodiment of the present invention. In the third and fourth embodiments, instead of forming the polarization beam splitter layer 23 on the hologram element 21, a triangular prism is used. In this case, a polarizing beam splitter layer 71 is formed on the slope of 70, and the surface of the hologram element 72 opposite to the grating layer is attached to the slope. As a method of attaching the hologram element 72 and the triangular prism 70, there is a method of injecting an adhesive. For example, when a transparent material having a refractive index equivalent to the refractive index of glass used for the prism 70, such as a photocurable resin, is used, the reflection loss between the hologram element and the prism is reduced, and the influence of refraction at the boundary is small. Become. The signal detection method is the same as described above.

[0027]

The birefringent diffraction grating polarizer of the present invention has a reflective structure, so that the manufacturing time can be shortened and the cost of the device can be reduced. Further, in the optical head device of the present invention, the size of the optical head device can be reduced because a reflection type birefringent diffraction grating polarizer is used. Further, by further modularizing the light source and the photodetector, the optical head device can be further improved. Miniaturization can be realized.

[Brief description of the drawings]

FIG. 1 is a sectional view showing a first embodiment of a birefringent diffraction grating polarizer of the present invention.

FIG. 2 is a sectional view showing a birefringent diffraction grating polarizer according to a second embodiment of the present invention.

FIG. 3 is a diagram showing a first embodiment of the optical head device of the present invention.

FIG. 4 is a diagram showing a second embodiment of the optical head device of the present invention.

FIG. 5 is a diagram showing a third embodiment of the optical head device according to the present invention.

FIG. 6 is a diagram showing a fourth embodiment of the optical head device according to the present invention.

FIG. 7 is a sectional view of a conventional hologram element.

FIG. 8 is a view showing a lattice pattern of a conventional hologram element.

FIG. 9 is a view showing a lattice pattern of a conventional hologram element.

FIG. 10 is a diagram showing a conventional optical head device.

FIG. 11 is a diagram illustrating a state of diffracted light incident on a photodetector.

FIG. 12 is a diagram showing a conventional optical head device.

FIG. 13 is a diagram illustrating a state of diffracted light incident on a photodetector.

FIG. 14 is a view showing a fifth embodiment of the optical head device according to the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Lithium niobate substrate 2 Proton exchange area 3 Phase compensation film 4 Total reflection film 5 Area A 6 Area B 7 Area C 8 Area D 9 Division line 10 Semiconductor laser 11 Collimating lens 12 Objective lens 13 Quarter wave plate 14 Optical disk 15 Magneto-optical disk 16, 17, 20, 21, 72 Hologram element 18 Polarization beam splitter 19 Lens 22 Reflection grating layer 23, 71 Polarization beam splitter layer 24 Mirror 25 Semiconductor laser chip 26, 27 Photodetection element group 30, 31, 32, 33, 34, 35, 36, 37 Photodetector 40, 41, 42, 43 Convergence point 50, 51, 52, 53, 54 Diffracted light 60, 61 Module 70 Triangular prism

Claims (5)

(57) [Claims] 1. A total reflection film is formed on a main surface of a crystal plate having optical anisotropy and on a surface of a diffraction grating comprising a periodically provided ion exchange region and a dielectric. Birefringent diffraction grating polarizer. 2. The birefringent diffraction grating polarizer according to claim 1, wherein a polarizing beam splitter layer is provided on the entire surface or a part of the surface opposite to the diffraction grating. 3. A light source, an imaging lens system for focusing light from the light source on a recording medium, and a converter for converting the polarization of light returned from the recording medium so as to be orthogonal to the polarization of light from the light source. A half-wave plate, a hologram element that reflects light from the light source to the imaging lens system and diffracts the return light, and a photodetector that receives the diffracted light; An optical head device, wherein a total reflection film is formed on a surface of a diffraction grating composed of a dielectric and an ion exchange region periodically provided on a main surface of a crystal plate having optical anisotropy. 4. A light source, an imaging lens system for focusing light from the light source onto a recording medium, and diffracting and reflecting return light from the recording medium by reflecting light from the light source to the imaging lens system. A hologram element, and a photodetector that receives the diffracted and reflected light, wherein the hologram element has an ion exchange region and a dielectric that are periodically provided on a main surface of a crystal plate having optical anisotropy. An optical head device, wherein a total reflection film is formed on the surface of a diffraction grating comprising: and a polarization beam splitter layer is provided in a partial region on a surface opposite to the diffraction grating. 5. A light source, an imaging lens system for converging light from the light source onto a recording medium, and a part of light from the light source reflected to the imaging lens system to return light from the recording medium A polarizing beam splitter that transmits a part of the light, a hologram element that reflects and diffracts return light from the recording medium that has passed through the polarizing beam splitter, and a photodetector that receives the reflected and diffracted light, In the polarization beam splitter, a polarization beam splitter layer is formed in a partial region of a surface where polarization is separated, and the hologram element is an ion exchange layer periodically provided on a main surface of a crystal plate having optical anisotropy. An optical head device, wherein a total reflection film is formed on a surface of a diffraction grating composed of a region and a dielectric, and an opposite surface of the diffraction grating is bonded to a polarization splitting surface of the polarization beam splitter.